The present invention relates generally to tiltable chairs, and in particular, to a synchrotilt chair having an adjustable seat, backrest and armrests.
Chairs of the type typically used in offices and the like are usually configured to allow tilting of the seat and backrest as a unit, or to permit tilting of the backrest relative to the seat. In chairs having a backrest pivotally attached to a seat in a conventional manner, the movement of the backrest relative to the seat can create shear forces which act on the legs and back of the user, and which can also create an uncomfortable pulling of the user""s shirt, commonly called xe2x80x9cshirt-pull.xe2x80x9d
To enhance the user""s comfort and to promote ergonomically healthy seating, synchro-tilt chairs provide for the seat and backrest to tilt simultaneously, but at different rates, preferably with the back tilting at a greater rate than the seat. Normally, synchro-tilt chairs employ compression and/or tension springs, torsion springs and/or torsion bars to bias the seat and back upwardly and to counterbalance the rearward tilting of the user. Chairs using these types of springs can have various limitations associated with the type of spring used therein.
For example, the proper placement of compression springs and/or torsion springs within the chair can often require a large or bulky housing with associated aesthetic limitations. Moreover, the ride, or resistive force experienced by the user, may be unsatisfactory because spring rates associated with compression springs are not linear and tend to increase as the spring bottoms out. In addition, the cost of manufacturing the chair, due to the placement of the springs and the introduction of additional load bearing elements, can be increased. This problem can be exacerbated when two or more springs are used in the chair. Moreover, synchrotilt chairs typically provide for the spring to act on one of the seat or back support, and for the force to then be transferred to the other through a pivotal attachment, which can require additional load carrying capabilities.
Furthermore, inconsistencies in the performance of compression and torsion springs, and the longevity thereof, can often be traced to the inherent properties of steel, which is typically used to make such springs. For example, steel is subjected to the problem of xe2x80x9ccreepxe2x80x9d and various inconsistencies introduced during the manufacture of the steel and the subsequent heat-treating processes. Moreover, because of the requisite size of the springs, the mechanisms used to adjust the amount of initial resistive compression can be difficult to activate, and can be progressively more difficult to adjust as higher settings are reached.
Chairs employing torsion bars may experience similar limitations. For example, the length and diameter of the bar is dictated by the range of movement and force output desired, and the desire to avoid overstressing the spring. Often, relatively heavy and highly stressed bars of great length are required to provide the control necessary to adequately support a user. Thus, the shape and associated aesthetics of the chair are dictated by the size of the spring. In addition, the chair must be provided with load-bearing elements at the ends of the bar and at the point of adjustment. Moreover, as with compression and torsion springs, activation or adjustment mechanisms used to achieve a desired initial pretorque setting can be difficult to manipulate, and can become increasingly so as higher settings are reached.
Leaf springs can also be used to support the user in the chair. However, leaf springs are typically clamped at one or more ends of the spring, usually by passing a bolt or like fastener through the spring. This is especially true when the leaf spring is configured as a cantilever similar to a diving board. Holes in the spring can introduce stress risers, however, and clamping one or more ends, as opposed to having them simply supported, introduces indeterminate moments and resultant stresses in the spring which may not be evenly distributed. Moreover, the resistive force of many leaf springs, including cantilevered springs, is often adjusted by varying the prestress of the spring through bending As with the other springs described above, such an adjustment mechanism can be difficult to activate, and becomes progressively more so as higher settings are reached.
It is also desirable to provide a chair that can be adjusted to accommodate the various needs and sizes of the user. In particular, it is desirable to provide a chair having an adjustable backrest, adjustable armrests, and an adjustable seat depth.
The typical approach to adjustably supporting a backrest is to provide a single, centered spline, which can be located internally or externally to the backrest cushion, or like support. Typically, such a spline is linear so as to allow for adjustment of the backrest. However, it is often desirable to provide contours in the backrest of the chair so as to conform to the shape of the user""s back. When the spline is located inside the backrest, the assembly is necessarily thick to accommodate the spline and desired contour. In addition, the backrest must itself be structural, and securely attached to the spline with tight tolerances, to provide lateral support for the user on the outer edges of the backrest and to avoid a feeling of sloppiness. Moreover, if armrests are desired, they must typically be positioned on separate supports projecting from the seat or from beneath the chair, since the spline centered backrest is usually structurally unable to support the large loads imparted on the armrests by a user along the sides of the backrest. When adjustable, such armrest supports often house complex and expensive to manufacture height adjustment mechanisms.
Furthermore, synchrotilt chairs typically provide pivot axes and links along the sides of the chair. Mechanically, there is an advantage to give the driven links input (occupant) and output forces (e.g., springs) as great a relative xe2x80x9cstancexe2x80x9d as possible. As a result, the use of a centered spline can result in a control that feels less xe2x80x9clivelyxe2x80x9d when the occupant is not centered. Additionally, centered spline chairs often provide an adjustment mechanism adjacent the spline at the center of the back, which can be difficult to access, especially by a seated occupant when the backrest is in a lowermost position.
Briefly stated, the invention is directed to an improved synchrotilt chair having an improved tilt control mechanism and an adjustable backrest, armrests and seat.
In one aspect of the invention, the chair includes a housing, a back support pivotally connected to the housing about a first horizontal axis and a seat support pivotally connected the housing about a second horizontal axis. A leaf spring includes a first end engaging a forward portion of the housing and a second end biasing the seat support and the back support in an upward direction. A fulcrum member is moveably supported in the housing and engages the leaf spring between its first and second end. In operation, the fulcrum member can be easily moved longitudinally within the housing so as to vary the length of the leaf spring lever arm and thereby vary the amount of resistive force supporting the user.
In a preferred embodiment, the seat support is also slideably connected to the housing about the second horizontal axis and is pivotally connected to the back support about a third horizontal axis.
In another aspect of the invention, a seat having a seat pan is adjustably mounted on the seat support. In operation, the seat can be moved in a longitudinal direction to adjust the depth of the seat relative to the backrest and thereafter releasably locked to the seat support.
In yet another aspect of the invention, a tilt limiter is provided to limit the rearward tilting of the chair. The tilt limiter includes a cam member pivotally mounted in the housing and having a plurality of teeth which engage a rack, or plurality of laterally oriented grooves, formed in the seat support. In operation, the cain member can be pivoted to limit the rearward tilting of the user
In another aspect of the invention, a selector member is connected to the tilt limiter. The selector member includes indicia that indicates the setting of tile tilt limiter so as to apprise the user of the maximum rearward tilt position of the seat, or chair, even when the seat or chair is in a tilt position other than the maximum rearward tilt position. In a preferred embodiment, the selector member comprises a handle connected to the cam member. The handle preferably has an substantially flat elongated portion forming the indicia such that the angular orientation of the substantially flat elongated portion indicates the setting of the tilt limiter, and the corresponding maximum rearward tilt position of the seat and chair.
In another aspect of the invention, the back support includes a pair of uprights extending upwardly along opposite sides of the chair. Each upright includes a first and second bar mounted thereto in a parallel and spaced apart relationship with the other. A backrest is slideably mounted on the first bar members and an armrest is slideably mounted to each of the second bar members. Preferably, the uprights are located externally of the backrest and are connected with a cross member so as to form a one-piece back support.
In a preferred embodiment, an engagement member is mounted to a bracket member which is mounted on the first bar member. The engagement member is adapted to engage a rack located on the upright to thereby releasably secure the backrest to the uprights. The armrest preferably includes a locking device which is adapted to engage the second bar member and thereby releasably secure the armrest to the upright.
The present invention provides significant advantages over other synchrotilt chairs, and chairs having adjustable backrests and armrests. For example, in the most preferred embodiment, an improved tilt control mechanism is provided which can be manufactured in a compact and aesthetically pleasing housing. In particular, the leaf spring, or preferably a pair thereof, extends longitudinally within the housing, which can be made in a compact and aesthetically pleasing form with little or no depth due to the nature of the spring. The width of the housing also need not be dictated by length of the spring. The resistive force of the leaf springs is easily and simply adjusted by moving the fulcrum member longitudinally within the housing. Consequently, the springs are not prestressed at differing levels, and the adjustment member can be easily manipulated without progressive difficulty The leaf spring also provides a relatively uniform spring rate throughout the tilting range of the chair.
The leaf springs also are preferably made of composite material, which is more resistant to creep. The leaf spring preferably supports a shaft pivotally connecting the seat support and back support. In this way, the leaf spring biases both members upwardly together, rather than acting on one member with the force then transmitted to the other member through a pivotal attachment. As such, the number of load bearing elements are reduced and simplified.
The three bar slide mechanism also provides several advantages. For example, the linkage provides for a synchrotilt chair wherein the back tilts at a greater rate than the seat, but avoids the use of a fourth bar, which can add to the complexity and manufacturing costs of the chair. Indeed, the overall design is greatly simplified by forming xe2x80x9cbarsxe2x80x9d out of the housing, seat support and back support. Additionally, the use of a slide member allows for the assembly to be made in a more compact and aesthetically pleasing form.
The unique back support also provides many advantages. For example, by providing a one-piece back support, a simplified and aesthetically pleasing structure is provided, which also performs the combined tasks of forming one of the bars of the linkage assembly, providing a support for the backrest and providing a support for the armrests. Additionally, the exoskeletal nature of the back support framing a cushion gives the user a strong visual of support, security and durability. Moreover, by providing uprights along the sides of the chair, the backrest is not required to be structural in nature, and the loads imparted by a user against the side of the backrest can be transmitted directly through the forwardly extending arms of the back support to the housing and spring member so as to provide a more xe2x80x9clivelyxe2x80x9d control for the user. Moreover, since the backrest is supported on both sides, looser tolerances can be accommodated during the assembly of the backrest without sacrificing any tightness in the feel of the backrest.
The uprights can also be used to also support the armrests, which thereby avoids the need for separate supports and complex mechanisms. In this regard, the bar members, which are mounted to the uprights, provide a simple but sturdy support for the backrest and armrests.
The present invention, together with further objects and advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.